(1) Field of the Invention
Our invention is a borehole probe used for measuring the tritium concentration in ground water.
(2) Description of the Prior Art
One of the radioactive waste products from nuclear power production is tritium or radioactive hydrogen of mass number 3. The release of radioactive waste products into rivers and streams introduces this tritium into the environment. Public and private agencies involved with nuclear regulation, the protection of the enivornment, and the study of water resources are concerned with the levels of tritium in our water supply. Presently, the technique for determining tritium levels in water is performed in the laboratory by first purifying the sample of obtained water and then releasing the tritium by electrolysis. Another technique employs calcium carbide to generate acetylene gas from the water sample. Our presently disclosed invention makes the separation of the sample site and incorporates the water purification previously done into one process.
Many devices are described in the prior art to detect the presence of radioactive isotopes including beta particle emitters such as tritium. These include liquid scintillation systems and gas proportional counters. In the gas proportional counter system, a voltage is applied across a chamber containing a special gas mixture that incorporates the tritium. Pulses of current which occur as the gas undergoes radioactive decay and ionization are counted to yield the desired result. The U.S. Pat. No. 2,470,221 (A. Scherbatskoy) discloses an ionization chamber system used in a borehole to detect radiations from radioactive materials in subsurface strata. Ions produced in the space 49 due to radiation entering the ionization chamber 30 will be detected at the alternating electric field (column 5, lines 58 et seq.). Such a system would be applicable to the measuring of natural gamma ray radiation coming from the rock surrounding the borehole because gamma rays can penetrate through the rock and the sonde casing. Tritium emits weak beta particles which could not penetrate such a casing. Our invention is sensitive only to these relatively low energy beta particles (0.026 million electron volts or MeV). Gamma rays are more easily detected by the Scherbatskoy device because of their high penetrating power and generally high energy (0.001 to 3 MeV) levels.
Deuterium has been used as an isotope tracer to measure the flow of ground water (U.S. Pat. No. 3,291,997, E. L. Albenesius et al). Deuterium, like tritium, is an isotope of hydrogen. However, unlike tritium, it is not radioactive. It is interesting to note that this inventor (see column 1, lines 40-43) thought that the weak radiation from tritium made it "essentially impossible to detect in situ."
None of the known prior art discloses systems that are capable of measuring the tritium content of water in situ as disclosed herein. U.S. Pat. No. 3,506,402 (H. F. Simon) discloses a technique for the analysis of organic compounds which can be marked or labeled with either radioactive carbon (.sup.14 C) or tritium (.sup.3 H). To accomplish its purpose, the organic compounds are broken up by a catalyst into compounds that do not condense on the walls of the ionization chamber. Methane gas is used as a carrier gas and the ionization chamber is a flow-through type. In our invention, the water is converted to a non-condensing material (acetylene) which acts as a carrier gas for the tritiated acetylene. We do not use a flowing gas in the ionization chamber as in Simon but instead use a fixed amount of gas which is introduced into the evacuated ionization chamber. The use of bulky apparatus such as that disclosed in Simon would be impractical in a borehole.
One previous patent which could be used to measure beta radiation in a borehole is disclosed in the S. B. Jones patent (U.S. Pat. No. 2,857,522). However, unless the tritium were present in very large concentrations, the phosphor (10) beta detector of Jones would be unsuitable. The metal or quartz protective window used by Jones would attenuate virtually all of the natural beta radiation striking it. Because the beta radiation emitted by tritium is so low in energy, it would be even more generally attenuated before reaching the phosphor. In contrast to the Jones patent, which is sensitive only to very high beta emitters, our invention is sensitive to very low concentrations. This is because the tritium is introduced directly into the counting chamber and hence the radiation experiences no attenuation.
What we have provided in our invention is an in situ low level tritium counting device which operates automatically to change water samples in the borehole, to eject the previous sample, and to insure the total program evaluation sequence.